1. Field of the Invention
The present invention relates to a production method of an optical recording medium. In particular, the present invention relates to a production method of a multilayer optical recording medium, in which a photocurable resin is applied to a substrate by employment of spin coating or the like so as to stack a recording layer, a light transmissive stamper for producing a multilayer optical recording medium, and a production method thereof.
2. Description of the Related Art
As information recording/reproducing technology with a high density, an optical recording medium such as a CD, DVD, and a Blu-ray (registered trademark) Disc (BD) is attracting a great deal of attention. In recent years, in association with a movement toward digitalization of a moving image and a movement toward miniaturization of an apparatus, there are demands for a higher recording density and a larger capacity.
In general, it is known that the recording density of an optical recording medium largely depends on the wavelength λ of a laser beam and the numerical aperture (NA) of an objective lens of a recording/reproducing optical system. In other words, the spatial frequency of recording pits capable of reproducing a signal is about 2 NA/λ. Accordingly, a number of studies have been conducted on the high recording density achieved by employment of a short wavelength technology or a high NA technology. For example, a CD has a recording capacity of about 650 MB when the wavelength of a laser beam for recording/reproduction is 780 nm and the NA of a lens is 0.45. A DVD-ROM has a capacity of about 4.7 GB when the wavelength of a laser beam is 650 nm and the NA of a lens is 0.6. Further, a BD has a capacity of more than 23 GB when the wavelength of a laser beam is as short as 405 nm and the NA of a lens is 0.85. Thus, the capacity of the optical recording medium has been increased.
On the other hand, more study has been conducted to realize a large capacity by stacking a plurality of recording surfaces. For example, a doubled recording capacity has been realized in, for example, a DVD and a BD, by stacking two layers of recording surfaces. With regard to the BD, a four-layered medium is under development aiming at higher integration to future generations.
Conventionally, as a production method of a multilayer optical recording medium, the following method has been employed. First, pits or guide grooves are formed in a surface of a substrate, and a reflective layer and a recording layer are formed thereon, to thereby form a first information recording layer.
After that, an ultraviolet curable resin (photocurable resin) or a dry photopolymer layer is formed on the first information recording layer (Step 1). A stamper which bears information (pits, guide grooves, or the like) necessary for the recording layer is superimposed on the ultraviolet curable resin or the dry photopolymer layer and cured so as to form pits or guide grooves, and the stamper is peeled (layer in which pits or guide grooves are formed is referred to as “intermediate layer”) (Step 2). A translucent film is formed on the pits or the guide grooves, to thereby form a second information recording layer (Step 3). The above-mentioned steps are repeated so as to form a third recording layer and a fourth recording layer (for example, see Japanese Patent Application Laid-Open No. 2003-203402 and Matsushita Technical Journal Vol. 50 (No. 5), October 2004, pp. 64-68).
In this case, when the ultraviolet curable resin or the dry photopolymer is cured through the recording layer (translucent film), there occurs an attenuation of the irradiated ultraviolet ray. Accordingly, in the above-mentioned proposal, it is necessary to use a stamper (light transmissive stamper) having transmissivity with respect to the ultraviolet ray and to irradiate the ultraviolet ray from the stamper side.
As the light transmissive stamper, a stamper made of a resin is usually used, but it is difficult to reuse the stamper made of a resin from the viewpoint of necessity of maintaining productivity and quality. Accordingly, the stamper is disposed of each time a layer is formed, which poses a problem of increase of the cost.
Further, a method of forming a pattern corresponding to an information pattern in a glass material by etching, which is used as a light transmissive stamper has been proposed (see, for example, Matsushita Technical Journal Vol. 50 (No. 5), October 2004, pp. 64-68).
The production method of the light transmissive stamper made of glass is illustrated in FIGS. 3A to 3E. First, a photoresist 32 is uniformly applied to a quartz glass substrate 31 (see FIG. 3A). Next, on the photoresist 32, a latent image corresponding to an information pattern is formed using a light beam 33 (see FIG. 3B). Then, a development process is performed so as to obtain the photoresist 32 that bears a desired pattern (see FIG. 3C). The quartz glass substrate 31 is subjected to dry etching from a surface of the photoresist 32 by use of a CF4 gas plasma 34, and the dry etching is stopped at a desired depth (see FIG. 3D). After that, the remaining photoresist 32 is removed by ashing with oxygen (O2), to thereby obtain the light transmissive stamper made of glass (see FIG. 3E).
In the above-mentioned method disclosed in Japanese Patent Application Laid-Open No. H01-188332, the light transmissive stamper can be repeatedly used. However, in the case of producing the light transmissive stamper, it is necessary to manage complicated processes including a dry etching process and an ashing process. In addition, CF4 gas and CHF3 gas used in the dry etching process are greenhouse effect gasses, and thus their use is restricted and undesirable. Further, in the method, the pattern is directly formed on the glass substrate, so it is necessary to polish and regenerate the glass substrate when the glass substrate is reused.